Barcelona Zoo has lost some animals I've been able to visit and really miss since their disappearance; black lemurs, black-and-white ruffed lemurs, mandrills, black-and-white colobus, common squirrel monkey, Tibetan macaque, lion-tailed macaque, siamang, Cuban crocodile (may be offshow, but I doubt it at this point), Morelet's crocodile, mugger crocodile, American alligator, Malaysian gharial, Red tegu, panther chameleon, leaf-tailed gecko, Cuban boa, yellow anaconda, reticulated python, African rock python, giant waxy monkey frog, fire-bellied toad, tomato frog, Chinese giant salamander, toco toucan, keel-billed toucan, Great Indian hornbill, rhinoceros hornbill, golden-headed quetzal, Congo grey parrot, turkey vulture, king vulture, Andean condor, common rhea, emu, Hawaiian goose, coscoroba swan, greater flamingo, golden eagle, common eland, greater kudu, gaur, American plains bison, collared peccary, nilgai, wapiti, waterbuck, sambar, Asian elephant, Southern tamandua, Eurasian lynx, snow leopard, cougar, African wild dog, Asiatic black bear, American black bear, harbour seal, bottlenose dolphin... The list could go on, but I'm gonna stop here XD
In all seriousness though, while I do miss these species, I do believe that a large part of them left for logical reasons, most notably for the optimization of the space the collection has available.